1. Field of the Invention
Embodiments of the present invention relate generally to integrated circuit chip packaging and, more specifically, to a packaging substrate that has electroplated pads that are free of plating tails.
2. Description of the Related Art
In the packaging of integrated circuit (IC) chips, an IC chip is typically mounted on a packaging substrate to facilitate electrical connection of the chip to a motherboard or other printed circuit board (PCB). The connection between the IC chip and the packaging substrate can be a plurality of solder joints, such as reflowed solder balls or solder micro-bumps, which are solder bumps deposited on surface pads of the IC chip. In order to provide a robust electrical connection between such a solder joint and a packaging substrate, it is generally desirable for the surface contacting the solder to be oxide-free. In package-on-package (PoP) design, in which one packaging substrate is electrically coupled to another packaging substrate, this is especially true, since the solder joints between the two packaging substrates may undergo multiple reflow (melting and solidification) processes.
Because the electrical connection pads formed on packaging substrates commonly include copper or a copper alloy, an oxidation layer can form on a pad surface before a solder joint is coupled to the pad. This layer of oxidized copper reduces the reliability of the solder joint formed on the pad surface and is therefore undesirable, particularly for solder joints coupling one packaging substrate to another in a package-on-package design. Consequently, a non-oxidizing surface finish is commonly deposited on electrical connection pads to prevent such oxidation, for example a layer of nickel-gold plating. Because such non-oxidizing surface finishes are deposited via an electroplating process, plating tails are necessarily formed on a surface of a packaging substrate prior to the electroplating process.
A plating tail is a conductive path or trace formed between a surface receiving the electroplated surface finish, such as an electrical connection pad, and a suitable connection point for an electrode. For example, plating tails are commonly routed to an easily accessible edge region of a packaging substrate to facilitate electrical coupling of the plating tails to an electrode during the electroplating process. Thus, for a packaging substrate that includes a large number of electrical connection pads to be plated with a non-oxidizing surface finish, a plurality of plating tails may be formed across an extensive area of the packaging substrate. In high-frequency circuits, the presence of such plating tails can degrade signal integrity in the circuit by increasing signal noise. For example, input signals to or output signals from an IC chip can reflect off the end of a plating tail or a step in a plating tail, thereby interfering with these signals and the proper operation of the IC.
As the foregoing illustrates, there is a need in the art for a packaging substrate that has electroplated pads that are free of plating tails.